Selenium dry-disk rectifier



Patented Dec. 22, 1953 UNITED STATES ATENT OFFICE SELENIUM DRY-DISK RECTIFIER Delaware No Drawing. Application February 9, 1951, Serial No. 210,288

Claims priority, application Germany February 14, 1950 1 Claim. 1

Selenium dry-disc rectifiers are basically built up in a way that on a base plate of some suitable material such as straight or nickel-plated iron, light metal, etc., there is applied a thin selenium coating on which a metallic back (counter) electrode of suitable composition is deposited.

Rectifier action is restricted to a thin layer which forms at the surface of the selenium facing the back electrode. The overall selenium coating must have a certain minimum thickness to avoid short-circuits between the base plate and the back electrode. The selenium layer proper is not identical however with the just mentioned thin rectifying layer forming at the selenium surface towards the back electrode. Strictly speaking, whatever selenium is not used in forming this barrier layer is superfluous, as it but causes a relatively high dropping resistance in the rectifying circuit so from the loss angle this portion of the rectifier should be made with a resistance as low as possible. This decrease in resistance or increase in conductivity is achieved by admixture to the selenium of iodine or other halogens in the form of chemical compounds or in their pure state. As time goes by, the selenium rectifier however displays a weakening of its properties called ageing. Experimentation now has revealed that this gradual deterioration is greatly slowed down by the addition to the selenium of certain admixtures. Oxides like aluminium oxide thus slow down the ageing process considerably. These additions may run as high as 150 milligram-percent. (1 milligram-percent means 1 milligram to 100 grams, or 0.00001 gram to 1 gram). The amount of admixtures is determined by the consideration that the conductivity must not be affected to any significant degree. Ageing of a selenium rectifier shows up as an increase in its forward resistance.

If one thus checks the value of this forward resistance by measuring the current under shortcircuit conditions, one will realize that the shortcircuit current through the rectifier decreases as time goes by. If however in accordance with the invention the aforenamed anti-ageing substances are added to the selenium, the decrease in the short-circuit current will be considerably slower through several thousand operating hours, so compared against rectifiers lacking the aforenamed additions, the same degree of deterioration will not take place before a considerably longer time, f. i. twice that time has elapsed.

Although so far one has no positive explana tion with regard to the mechanism of the action of these additions, one can presume that it is based on the following: rectifier ageing is dependent upon temperature. This fact induces one to presume that the iodine in the selenium moves gradually away into the barrier layer due to difiusion phenomena. This however means an increased resistance in that portion of the selenium film not used as a barrier layer. The eifect of the aforementioned additions now may base upon the fact that the iodine is retained within the selenium so the ageing properties are improved while the diffusion phenomena are slowed down. Whatever the true mechanism, experimentation discloses that upon the addition of these substances, the ageing process is con siderably delayed.

I claim:

A selenium rectifier comprising a base plate, a layer on said base plate consisting of selenium containing admixtures of a halogen and aluminum oxide in which the aluminum oxide is present in said layer up to milligram-percent of the selenium, and a counter-electrode on said layer.

OTTO J. KLEIN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,137,316 Van Geel et a1 Nov. 22, 1938 2,189,576 Brunke Feb. 6, 1940 2,453,763 Smith Nov. 16, 1948 

